Weird amp whistle problem

I have an Ampeg SVT3pro run through an SVT classic 2x10 and 1x15 stack. Last night during a set we played Hot Hot Hot and I cranked the amp a bit for the slap line. All of a sudden I got this high pitched whistle like feedback but kind of wavering in pitch. I turned down and it went. Is this a common ailment? Do you know what it is?
I didn't even know it was me at first. I thought it was the PA and we were very loud at the time so it was hard to locate. But I am pretty sure it was me.

I had a similar problem with a combo - the same noise happened while playing in church and the sound people were freaking out trying to find it. One of the members in the congregation saw one of my speakers lighting up and pointed it out. Some kind of arcing I guess. I thought it was coming from somewhere else (I guess bass isn't directional, so it seems to come from all over...). I wasn't playing all that loud, either. During sound check it was fine, but the noise started on the first song of our set. Took it home and tried to replicate it, but the thing worked fine...

saw one of my speakers lighting up and pointed it out. Some kind of arcing I guess.

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No no no. There is a light bulb inside your speaker cabinet, which lights up to absorb any excess current to the tweeter. This is a protection mechanism. There was no arcing, it was simply doing what it was designed to do.

The whistle could have been several things. Pretty much all of them are related to the total gain of the system, affected by volume, master, etc.

Feedback to a microphonic tube in the preamp.

Feedback to another microphonic part. Capacitors are known for being or becoming microphonic.

Feedback from speaker cables to instrument or cable, magnetically or otherwise.

Regular acoustic feedback to the instrument.

Or you could have something loose inside the amp, which doesn't cause a problem, but makes the amp more sensitive to internal feedback. Loose shielded cable, ground connection going bad, etc.

I once worked on a rig for a local Illinois band in the 70's. The bass player had an Alembic preamp, and a big Crown amp. He ran one channel of the pre into the other, in series, and turned both to max.... The system was just plain unstable. Not a cable made that would fix that much gain....and the Alembic wasn't designed to be run flat out in series. It had nearly the gain of a booked Mesa guitar amp...... But Mesa were rolled off, and the Alembic was flat. The old Mesa amps weren't very stable, and no way the Alembic was gonna be either.

He wasn't happy, but there wasn't anything we could do that wouldn't mess up the tone of the system. just too much gain.

I doubt you ran into that, but it is the "limit case" of the problem you had.

Notanaggie covered a lot of it. I would add that getting close to the speakers with non-hum-bucking coils can give squeals from a direct electronic feedback also. But usually you are already getting acoustic feedback then.

Notanaggie: Was that bass player's name Marty? I think I saw that setup, now I'm trying to remember the band name......they were pretty big locally, IIRC....